Various types of pumps are commonly used to pump fluids from one location to another. Liquid metal, or another fluid of sufficiently high electrical conductivity, can be pumped by means of an electromagnetic (EM) pump. A description of the theory behind and applications of such pumps can be found in the McGraw-Hill Encyclopedia of Science & Technology, 6th Edition, Volume 6, pages 153-54, published by McGraw-Hill, Inc. EM pumps are based on the principle that a force is exerted on a current-carrying conductor when it is placed in a magnetic field. There are several types of EM pumps, with the various designs being differentiated by the character of the current (direct or alternating current) and magnetic field used, and the method used to cause current to flow within the pumped fluid.
One of the most common types of EM pumps is termed a conduction pump, which may be of either the direct current (dc) or alternating current (ac) variety. A dc conduction EM pump consists of a pump duct or conduit through which flows a conductive fluid (such as molten metal) and across which is applied a magnetic field in a direction perpendicular to the desired direction of fluid flow. The magnetic field may be produced by either a permanent magnet or an electromagnet. Electrodes are arranged such that they form a path for current to be conducted through the fluid in a direction perpendicular to both the desired direction of fluid flow and the direction of the magnetic field. The relationship between the current, magnetic field, and intended direction of fluid flow subjects the fluid to a force which acts to pump it through the conduit (where the incremental force dF is given by dF=I dl.times.B, where dl is the increment of length of the current path in the fluid and B is the magnetic field).
The physical arrangement of the elements in such a pump allows only a single passage of current between the electrodes, resulting in the existence of a relatively short length for the current path within the fluid. To compensate for this situation, and to increase the force exerted on the fluid, very high currents (typically thousands of amperes) at low voltages (typically less than 1 volt) are used. The high currents can result in the production of heat due to the I.sup.2 R power dissipation occurring within the conducting fluid. This situation, and the need to protect the magnet from the fluid, which itself may be at a high temperature, means that the magnet is usually both electrically and thermally insulated from the conduit.
Standard EM pumps have several advantages over mechanical pumps. The absence of moving parts within the pumped fluid means that seals or bearings are not needed. This minimizes required maintenance and improves reliability. EM pumps can also operate in a vacuum, something that some mechanical pumps are not capable of doing.
However, EM pumps also have several disadvantages. Because the conduit is often relatively flat in order to increase the magnitude of the force applied to the conducting fluid, such pumps are prone to becoming obstructed. The magnet used to create the magnetic field must be close in proximity to the conduit and electrically and thermally insulated from it. This makes the pump bulkier and complicates its design and fabrication. EM pumps also require a power supply to provide the conduction current and may require an additional power supply if an electromagnet is used to produce the magnetic field.
In addition, although EM pumps are capable of pumping large volumes of fluid (hundreds to thousands of gallons per minute), because such pumps are sensitive to the head depth of the pump, they are not as easily used to pump small amounts or to regulate the rate at which fluid is pumped. This can be a disadvantage in circumstances where it is important to be able to precisely control the amount of fluid which is pumped, or to obtain a highly reproducible pump rate. Finally, the fluid which is to be pumped must have a sufficiently high electrical conductivity in order for the pump to be effective. This limits the applications for which the pump is suited.
Another type of pump which can be used to pump fluids is a bubble or air-lift pump. This type of pump uses gas (typically air) forced under pressure into a cavity to create a stream of bubbles, which carry liquid with them as they rise up a pump tube. A theoretical analysis of the performance of air-lift pumps is contained in the article "An Analytical and Experimental Study of Air-Lift Pump Performance", by A. H. Stenning and C. B. Martin, published in Transactions of the ASME, Journal of Engineering for Power, pp. 106-110, April 1968.
Air-lift or bubble pumps are commonly used for purposes of underwater exploration and mining. However, standard bubble pumps are not suitable for pumping fluids in a vacuum because as the bubbles burst, the quality of the vacuum will be reduced. This limits the usefulness of bubble pumps, as it is desirable to pump fluids such as liquid metal in a vacuum in order to reduce the reactivity of the molten metal. Pumping molten metal in a vacuum serves to lessen damage to the pump and limits chemical reactions involving the metal which may reduce its usefulness for the purpose for which it is intended.
What is desired is an apparatus and method for pumping high temperature molten metal or other fluids in a vacuum or low pressure environment, which is not subject to the disadvantages of pumps currently used for such purposes.